Socket head cap screws are used in a variety of applications and it is frequently desirable to cover the head of the screw. A plastic accessory is often used for this purpose and may take a multitude of forms, e.g., a dial, insulator, decorative cover, adjustment foot and a thumb screw. The plastic accessory may also be formed as a knob permitting the cap screw to be tightened or loosened by hand.
While plastic accessories used in combination with a variety of screws, are known in the art, they are typically molded onto the screw or adhesively attached thereto. The torque requirements of the plastic accessories, however, not only serve to make the requisite molding or adhesives costly but also necessitate expensive and specialized tooling.
Thus, it is desirable to have a plastic accessory which not only may be securely attached to the head of a socket head cap screw, but also is functionally capable of withstanding a broad range of screw tolerance specifications. Further, in consideration of the cost and ease factors as previously discussed, it is desirable to attach the plastic accessory without adhesives or special tooling. The plastic accessory must, however, be capable of withstanding and producing, without failure or deterioration, the requisite mechanical torque. An example of such a plastic accessory is disclosed and described in U.S. Pat. No. 3,529,508.
In recent years, the manufacturing techniques for, and specifications of, socket head cap screws have changed. For example, minor diameter tolerances are now absorbed near the screw head creating a back draft portion thereby decreasing the screw head and plastic accessory contact area. Additionally, it has become more typical in manufacturing to add a lead-in angle to the drive socket portions of the screw head. The lead-in angle alters the engaging characteristic and subsequently the torque characteristic of the plastic accessory.
With respect to specifications, the standards to reference for manufacturing socket head cap screws require that a plastic accessory overcome the head diameter tolerances while still maintaining acceptable torque and connections between the screw and plastic accessory. For example, the latest Japanese Industrial Standard (JIS-B-1176-1988) as revised in 1988 for an M10 hexagonal socket head cap screw has a head diameter which may vary from 15.73 mm to 16.27 mm. This variance corresponds to an additional 0.27 mm or 60% more permissible head diameter tolerance the accessory retaining socket must now overcome. The existing M10 United Kingdom standard for a hexagonal socket head cap screw provides that the head diameter may vary from 15.57 mm to 16.00 mm and the M10 American Standard has a head diameter variance of 15.67 mm to 16.00 mm. Additionally, the sooket head heiqht, the key engagement dimension and the concentricity of the socket within the head cap screw are all subject to manufacturing variances and tolerances.